BlaB! AX is a free AJAX/PHP group chat system. Both the frontend and Admin CP are with responsive design, HTML5 and CSS3 compliant, support all modern browsers (incl. mobile browsers), HiDPI displays ready (no raster images).
BlaB! AX is tiny - all that users load with their browsers is less than 200kB (60kB gzipped) in 5 HTTP requests (incl. sounds, emoticons and other images; stickers and avatars are loaded on request) and 95% of this content is cached for further use.

Rooms - separate chat spaces. Every chatter is connected to all the rooms and receives any message posted in any room with a visual notification and a different sound notification for messages posted in background rooms.

Private one-2-one chats inside the main chat.

Admin can kick and ban users and IP addresses.

Material icons, EmojiOne emoticons, native emojis & stickers.

Colorful predefined themes; option to create your own in seconds.

Two ready to use Android apps available on Google Play.

An easy way to create your own custom mobile app for Android and IOS.

An easy way to create a Chrome web extension /see FAQ/.

Ignore option - chatters can choose to hide messages from other chatters.